1. Field of the Invention
The present invention relates to an efficient digital transmission technique, and more particularly, a method and apparatus for modulating digital data.
2. Description of Related Art
Digital communication relies on numerous different, albeit related, forms of digital modulation such as phase shift keying (PSK), bi-phase shift keying (BPSK), quadrature phase shift keying (QPSK or 4-PSK), and quadrature amplitude modulation (QAM).
BPSK will be described with reference to FIG. 1. As shown, the magnitude of a reference carrier is constant, and to transmit either a 0 or a 1, the phase thereof is xe2x80x9ckeyedxe2x80x9d or switched between 0xc2x0 and 180xc2x0. A receiver then decides whether a 0 or a 1 was transmitted based on the phase of the received carrier, and generates the original data stream. With this simple scheme, one bit of information is transmitted with each state or symbol, so that the carrier phase is keyed at the data rate. FIG. 1 also illustrates the constellation for BPSK. As shown, the BPSK constellation diagram includes two points in the I-Q plane where I stands for in-phase (i.e., phase reference) and Q stands for quadrature (i.e., 90xc2x0 out-of-phase). The two points in the BPSK constellation diagram represent the position of the signal at the xe2x80x9ctiming instancexe2x80x9d. The timing instance is when the receiver interprets the signal. The signal can only be at one position at a time, but the constellation can be thought of as having persistence so that all of proper the states appear. Constellation diagrams such as in FIG. 1 typically do not show the transition between states and it should be noted that this transition does take a finite time. But for clarity, the transitions are not shown otherwise traces connecting the two states would clutter the diagram.
FIG. 2 illustrates the constellation diagram for QPSK. As shown, four different states exist in the QPSK diagram at phase values of 45xc2x0, 135xc2x0, 225xc2x0, and 315xc2x0. As further shown, each state corresponds to a symbol representing two bits. Because the data is taken two bits at a time to form a symbol, the symbol rate is half the bit rate. As a result, QPSK requires half the band width of BPSK for the same bit rate.
Transmission of the modulated signal usually requires generating an amplified modulated signal. Unfortunately, amplification can introduce distortion, which alters the bandwidth of the signal. Many multiple access communication techniques, such as time division multiple access (TDMA), require maintaining transmitted signals within a narrow frequency band to increase capacity. For this reason, linear amplifiers have generally been used to amplify modulated signals when such multiple access communication techniques are employed. Linear amplifiers, while amplifying the modulated signal, also preserve the frequency of the signal, and give a better narrow band accuracy. By narrow band, it is meant that the frequency band of the carrier signal is larger than the frequency band of the signal.
Linear amplifiers, however, consume significant power. Power becomes an issue, particularly, in wireless communication systems where the individual mobile stations are powered by a limited power source. Accordingly, techniques that advantageously lengthen the life of such power sources are highly desirable.
The method and apparatus for modulating digital data according to the present invention uses non-linear amplifiers. Non-linear amplifiers, occasionally called switching or high frequency amplifiers and commonly known (but not limited to) as classes D, E, F, G and H, faithfully reproduce the phase of a signal, but not the envelope (in contrast to linear amplifiers). Non-linear amplifiers consume significantly less power than linear amplifiers, but because of the distortion they introduce, typically do not meet narrow frequency band requirements. However, the method and apparatus for modulating digital data according to the present invention, while using non-linear amplifiers, also meets narrow frequency band requirements.
The modulator includes an oscillator generating signals, each having a different phase. A selector, in the modulator, sends a first plurality of these signals to a first non-linear amplifier when the digital data received by the selector changes logic state (e.g., goes from 0 to 1 or from xe2x88x921 to 1), and also sends a second plurality of the signals to a second non-linear amplifier when the digital data changes logic state. The outputs of the first and second non-linear amplifiers are summed to generate a radio frequency output that does not suffer from distortion typically introduced by non-linear amplifiers.